> -----Original Message----- > From: Alexander Graf [mailto:agraf@xxxxxxx] > Sent: Wednesday, April 03, 2013 7:39 PM > To: Bhushan Bharat-R65777 > Cc: Wood Scott-B07421; kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx > Subject: Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub support > > > On 03.04.2013, at 15:50, Bhushan Bharat-R65777 wrote: > > > > > > >> -----Original Message----- > >> From: kvm-ppc-owner@xxxxxxxxxxxxxxx > >> [mailto:kvm-ppc-owner@xxxxxxxxxxxxxxx] On Behalf Of Alexander Graf > >> Sent: Wednesday, April 03, 2013 3:58 PM > >> To: Bhushan Bharat-R65777 > >> Cc: Wood Scott-B07421; kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx > >> Subject: Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub > >> support > >> > >> > >> > >> Am 03.04.2013 um 12:03 schrieb Bhushan Bharat-R65777 <R65777@xxxxxxxxxxxxx>: > >> > >>> > >>> > >>>> -----Original Message----- > >>>> From: Wood Scott-B07421 > >>>> Sent: Tuesday, April 02, 2013 11:30 PM > >>>> To: Bhushan Bharat-R65777 > >>>> Cc: Alexander Graf; kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; > >>>> Wood Scott- > >>>> B07421 > >>>> Subject: Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub > >>>> support > >>>> > >>>> On 04/02/2013 09:09:34 AM, Bhushan Bharat-R65777 wrote: > >>>>> > >>>>> > >>>>>> -----Original Message----- > >>>>>> From: Alexander Graf [mailto:agraf@xxxxxxx] > >>>>>> Sent: Tuesday, April 02, 2013 1:57 PM > >>>>>> To: Bhushan Bharat-R65777 > >>>>>> Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; Wood > >>>>>> Scott-B07421 > >>>>>> Subject: Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub > >>>>> support > >>>>>> > >>>>>> > >>>>>> On 29.03.2013, at 07:04, Bhushan Bharat-R65777 wrote: > >>>>>> > >>>>>>> > >>>>>>> > >>>>>>>> -----Original Message----- > >>>>>>>> From: Alexander Graf [mailto:agraf@xxxxxxx] > >>>>>>>> Sent: Thursday, March 28, 2013 10:06 PM > >>>>>>>> To: Bhushan Bharat-R65777 > >>>>>>>> Cc: kvm-ppc@xxxxxxxxxxxxxxx; kvm@xxxxxxxxxxxxxxx; Wood > >>>>> Scott-B07421; > >>>>>>>> Bhushan > >>>>>>>> Bharat-R65777 > >>>>>>>> Subject: Re: [PATCH 4/4 v2] KVM: PPC: Add userspace debug stub > >>>>>>>> support > >>>>>>>> > >>>>>>>> > >>>>>>>> How does the normal debug register switching code work in Linux? > >>>>>>>> Can't we just reuse that? Or rely on it to restore working > >>>>>>>> state > >>>>> when > >>>>>>>> another process gets scheduled in? > >>>>>>> > >>>>>>> Good point, I can see debug registers loading in function > >>>>> __switch_to()- > >>>>>>> switch_booke_debug_regs() in file arch/powerpc/kernel/process.c. > >>>>>>> So as long as assume that host will not use debug resources we > >>>>> can rely on > >>>>>> this restore. But I am not sure that this is a fare assumption. > >>>>>> As > >>>>> Scott earlier > >>>>>> mentioned someone can use debug resource for kernel debugging also. > >>>>>> > >>>>>> Someone in the kernel can also use floating point registers. But > >>>>> then it's his > >>>>>> responsibility to clean up the mess he leaves behind. > >>>>> > >>>>> I am neither convinced by what you said and nor even have much > >>>>> reason to oppose :) > >>>>> > >>>>> Scott, > >>>>> I remember you mentioned that host can use debug resources, you > >>>>> comment on this ? > >>>> > >>>> I thought the conclusion we reached was that it was OK as long as > >>>> KVM waits until it actually needs the debug resources to mess with the > registers. > >>> > >>> Right, Are we also agreeing on that KVM will not save/restore host > >>> debug > >> context on vcpu_load/vcpu_put()? KVM will load its context in > >> vcpu_load() if needed and on vcpu_put() it will clear DBCR0 and DBSR. > >> > >> That depends on whether the kernel restores the debug registers. > >> Please double- check that. > > > > Currently the kernel code restore the debug state of new schedule process in > context_switch(). > > > > switch_booke_debug_regs() from __switch_to() and defined as : > > /* > > * Unless neither the old or new thread are making use of the > > * debug registers, set the debug registers from the values > > * stored in the new thread. > > */ > > static void switch_booke_debug_regs(struct thread_struct *new_thread) > > { > > if ((current->thread.dbcr0 & DBCR0_IDM) > > || (new_thread->dbcr0 & DBCR0_IDM)) > > prime_debug_regs(new_thread); } > > > > static void prime_debug_regs(struct thread_struct *thread) { > > mtspr(SPRN_IAC1, thread->iac1); > > mtspr(SPRN_IAC2, thread->iac2); #if CONFIG_PPC_ADV_DEBUG_IACS > > > 2 > > mtspr(SPRN_IAC3, thread->iac3); > > mtspr(SPRN_IAC4, thread->iac4); #endif > > mtspr(SPRN_DAC1, thread->dac1); > > mtspr(SPRN_DAC2, thread->dac2); #if CONFIG_PPC_ADV_DEBUG_DVCS > > > 0 > > mtspr(SPRN_DVC1, thread->dvc1); > > mtspr(SPRN_DVC2, thread->dvc2); #endif > > mtspr(SPRN_DBCR0, thread->dbcr0); > > mtspr(SPRN_DBCR1, thread->dbcr1); #ifdef CONFIG_BOOKE > > mtspr(SPRN_DBCR2, thread->dbcr2); #endif } This is analogous to > > moving from guest to/from QEMU. so we can make prime_debug_regs() available to > kvm code for heavyweight_exit. And vcpu_load() will load guest state and save > host state (update thread->debug_registers). > > I don't think we need to do anything on vcpu_load if we just swap the thread- > >debug_registers. Just make sure to restore them before you return from a heavy > weight exit. My understanding is : 1) When VCPU is running -> h/w debug registers have vcpu->arch.debug_registers Goes for heavyweight_exit -> h/w debug registers are loaded with thread->debug_registers Return from heavyweight_exit and run vcpu -> h/w debug registers have vcpu->arch.debug_registers. Do we need to save thread->debug_registers , probably no as the thread->debug_registers are upto date. 2) KVM to kernel -> clear DBSR and DBCR0 Kernel to KVM -> load vcpu->arch.debug_registers. > > > And the kernel exception handling code clear DBSR and load DBCR0 with > > 0 (global_dbcr0 variable, which is zero) in transfer_to_handler in entry_32.S > This is analogous to switching from KVM to kernel. > > But I do not same (clearing DBCR0 and DBSR) in 64bit exception handler. Is > this a problem or I am missing something. > > That one's up for the normal ppc ML to answer :). Will take this on ppc mailing list. Thanks -Bharat > > > Alex > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
